Needle valve nozzles are installed in injection moulding tools in order to feed a flow-capable mass to a separable mould insert at a specifiable temperature and under high pressure. They are usually equipped with pneumatically or hydraulically driven shut-off needles, which periodically open and close the sprue openings in the mould insert. For this purpose, every shut-off needle is positioned in the tool area of the injection moulding device in an axially displaceable manner and preferably guided through the center of a flow channel in the nozzle area for the mass to be processed. The flow channel terminates in a nozzle end piece that forms a spray orifice. In a closed position, the lower end of the shut-off needle engages with a landing bar that is formed in the nozzle end piece or the mould insert.
In order to be able to synchronously open and close several needle valve nozzles in one tool, it is possible to follow a procedure familiar from EP-A2-0790 116 to fasten the shut-off needles along a common carrier plate, which carries out a displacement motion in the longitudinal direction of the shut-off needles. For this purpose the carrier plate is arranged on the front between two fixed stops and on the side between two guide strips, which are positioned inside a bolster plate in a longitudinally displaceable manner and have oblique sliding blocks or sliding cams on the side surfaces that are directed to the carrier plate. The sliding cams engage with the side of the carrier plate, which is furnished with obliquely running grooves. If the guide strips are moved back and forth in the longitudinal direction by means of an actuator, the carrier plate moves up and down accordingly in a vertical direction. This way all shut-off needles fastened to the carrier plate will carry out the same displacement motion.
This kind of operating device has the disadvantage that the dimensions of the carrier plate and the number of shut-off needles fastened to it have fixed specifications. Therefore any extensions are only possible by replacing the carrier plate and guide strips, which is associated with a substantial effort and corresponding costs.
In an injection moulding tool referred to by DE-A1-196 11 880 with several needle valve nozzles, each shut-off needle is fastened to a separate needle support element. These are equipped with obliquely positioned guiding cams positioned on two opposing flat surfaces, which engage with the obliquely running grooves of a fork-shaped sliding frame. A cylindrical section is formed at each needle support element below the flat surfaces, which is positioned just like a piston in a guide bushing in an axially displaceable manner. When the sliding frames are moved back and forth, the individual needle support elements move up and down accordingly in a vertical direction.
This too only allows for a limited amount of needle support elements to be arranged in succession. Each additional element requires an extended sliding frame which has to be fabricated separately. It is also problematic that every shut-off needle has to be attached to its needle support element by way of an elastic interim element, which also increases the maintenance efforts and fabrication costs.
DE-A1-199 07 116 reveals an operating mechanism for injection moulding valve elements that is positioned between two tool plates. The individual valve bolts of a nozzle group are attached to a common valve bolt plate, which is equipped with guide bushings along the edge and can glide up and down on guide bolts that are parallel to the valve bolts. Two actuating arms are attached to the valve bolt plate, which support several sliding blocks on the side. These engage with the obliquely running grooves of two cam elements, which are each positioned in a longitudinally displaceable manner between the upper tool plate and one mounting plate each.
Since the intervals between the guide bolts and the position of the actuating arms are clearly specified, no subsequent changes may be made here either. Additional shut-off needles can only be attached if the entire valve bolt plate is replaced, which is associated with a major construction expenditure. The placement of the actuating arms and the additionally required mounting plates also result in a relatively large overall height of the operating device, which means that the use of the operating device is limited, especially in smaller tools.